class_docstr: Attach validator to the trainer engine in Ignite.
  It can support to execute validation every N epochs or every N iterations.
docstr: Args:
      engine: Ignite Engine, it can be a trainer, validator or evaluator.
docstr_tok: ["Args",":","engine",":","Ignite","Engine",",","it","can","be","a","trainer",",","validator","or","evaluator","."]
code: def __call__(self, engine: Engine) -> None:
      """
      Args:
          engine: Ignite Engine, it can be a trainer, validator or evaluator.
      """
      if self.validator is None:
          raise RuntimeError(
              "please set validator in __init__() or call `set_validator()` before training."
          )
      self.validator.run(engine.state.epoch)
  
file_path: MONAI/monai/handlers/validation_handler.py
imports: from __future__ import annotations
  from typing import TYPE_CHECKING
  from monai.config import IgniteInfo
  from monai.engines.evaluator import Evaluator
  from monai.utils import min_version, optional_import
label_desc: Use image node_id 4 for calling the ValidationHandler obj's underlying member method code with example usage: obj.__call__(engine) without return types